One type of commonly used flow control valve incorporates a diaphragm mounted within the valve and with one face of the diaphragm exposed to the fluid flow path through the valve and the opposite face exposed to a closed pressure chamber. An aperture is formed through the diaphragm so that fluid pressure in the valve flow path is transmitted to the opposite face of the diaphragm and, because the area of the opposite face of the diaphragm is larger than the face exposed to the flow path, a greater total pressure developes on the pressure chamber side of the diaphragm, forcing it to a closed position.
To open the valve a passage connecting the pressure chamber to the downstream side of the flow path is opened to allow the pressure in the pressure chamber to decrease and the diaphragm to move from its closed to its opened position.
The size of the aperture through the diaphragm will control the rapidity with which the diaphragm operates to open and close the valve. While a reasonably fast acting valve is desirable, it will be appreciated that damage to the lines with which the valve is associated can occur if the aperture in the diaphragm is too large, since pressure build ups will occur too rapidly and cause the diaphragm to slam into the closed position. However, the aperture cannot be made too small because it would then be susceptible to clogging by either the build up of corrosion on the wall of the aperture or the presence of foreign particles in the valve.
To overcome these problems conventional practice is to provide a fairly large size aperture through the diaphragm and then decrease the open area of the aperture by means of a metering rod which is fixed in the valve and extends through the diaphragm aperture.
In this way, the diaphgram aperture can be formed fairly large without rapid pressure build ups and possible damage to the valve and the movement of the diaphgram while the metering rod remains fixed tends to keep the diaphgram aperture free from clogging by either corrosive build up or foreign particles.
However, the diaphgram does not always move uniformly between its open and closed positions. In other words, the diaphgram may lift from its seat at one area before it does at another area rather than moving uniformly off its seat at all portions of its periphery. This non-uniform movement of the diaphgram can result in the metering rod binding in the aperture opening, particularly since the clearance between the rod and the diaphgram aperture is rather small, and result in the valve being stuck in the open position.
Thus, while the use of a metering rod avoids the problems noted above with regard to the rapidity of the closing of the valve and the clogging of the diaphgram aperture, it will be seen that it nonetheless presents the additional problem of binding which can cause the valve to malfunction in the open position.